This invention relates to copending Serial No. 871,444 filed June 6, 1986, and the disclosure thereof is incorporated herein by reference.
This invention pertains to thermosetting polymeric binders useful in protective coatings and particularly to a transesterification cure catalyst comprising minor amounts of an epoxy resin in combination with an inorganic salt selected from halogens, azides, cyanides, and hydroxides to actively promote transesterification coreaction between matrix polymers containing reactive hydroxyl groups and polymers containing coreactive ester groups.
Transesterification of a simple ester compound with a simple alcohol compound is known to occur under basic conditions. The transesterification reaction is an equilibrium reaction which can be driven to completion by removing the alcohol moiety evolving from the cleaved ester. If the cleaved alcohol moiety is a low molecular weight lower alkyl alcohol such as methanol or ethanol, removal by evaporation is quite easy. It has been found that transesterification as a curing mechanism for crosslinking polymers used in paint coatings provides an attractive cure mechanism for producing thermosetting protective coatings since cleaved lower alkyl alcohols are easily removed from the coating by simple air dry evaporation thereby driving the transesterification reaction to completion.
Several patents disclosed the use of acids, bases, metal salts, and organic metal complexes as catalysts for transesterifying polymers such as U.S. Pat. Nos. 4,362,847, 4,376,848, 4,332,711, and U.S. Pat. No. 4,459,393 wherein octoates or naphthenates of lead, zinc, calcium, barium, and iron are disclosed as transesterification catalysts.
Green U.S. Pat. No. 4,559,180 discloses an organic syntheses process for simple transesterification of very low molecular weight simple carboxyl ester compounds with similar simple alcohols by reacting the same in the presence of an epoxide and a Lewis base (containing a Group V element) or a cyclic amidine. The Green patent does not pertain to coatings or polymers but merely discloses simple chemical reactions between very low molecular weight chemical compounds.
Dante and Parry have shown that phosphonium halides, such as ethyltriphenyl phosphonium iodide, are efficient catalysts for (a) 1,2-epoxide reactions with phenols to yield hydroxy ethers (U.S. Pat. No. 3,477,990), and (b) polyepoxide reactions with carboxylic acids or acid anhydrides (U.S. Pat. No. 3,547,885). The patent suggest that polyepoxides and phenols can be reacted to form phenolic hydroxy ethers with phosphonium salts as catalysts. The counterion of the phosphonium moiety is the anion portion of a carboxylic acid, or acid ester, such as in ethyltriphenyl phosphonium acetate (U.S. Pat. No. 3,948,855).
Barnhoorn et al (U.S. Pat. No. 4,459,393) teach self-crosslinking thermosetting resin compositions obtained from the reaction of a beta-hydroxyalkyl ester of an alpha,beta-carboxylic acid with a primary mono- or polyamine to give a product having 1 to 2 amino hydrogens and further reacted with a polyglycidyl ether of a polyhydric phenol so that the final resin adduct has more than one beta-hydroxyalkyl ester group and amine groups having 1 to 2 amine hydrogen atoms per molecule. Transesterification catalysts known in the art are taught.
Subramanyam et al (U.S. Pat. No. 4,376,848) teach the preparation of water dilutable electrocoating compositions having tertiary amino-containing basic binders by reacting a secondary amino group compound with an olefinically double-bonded epoxy and the copolymerization of this produce with at least one ethylenically bonded polymerizable monomer wherein said binders can self-cure and be cured in combination with amine resins and/or phenolic resins. Again, common transesterification catalysts are taught.
It now has been found that a transesterification catalyst combination of an inorganic salt of a halide, an azide, a cyanide, or a hydroxide and a minor amount of a catalytic epoxy compound containing oxirane functionality efficiently, activates transesterification thermosetting cure reaction between a polymer containing a carboxyl ester linkage and a polymer containing an alcohol group. The cation preferably is a monovalent inorganic element selected from sodium, potassium, lithium, calcium or iron. Polyvalent cations such as calcium and iron are also useful. Coreactive ester polymers and hydroxyl polymers can be efficiently crosslinked or cured at room temperature or above room temperature in accordance with this invention. The curing mechanism satisfies an ongoing need in the coatings field for improved and more efficient curing mechanisms which are less toxic and conform to VOC standards. This invention overcomes dificiences of current crosslinking systems based on amine resin/aldehyde condensation compositions reactive with hydroxyl and other acidic functionality which exhibit a number of deficiences including high cure temperatures, pH dependence, formaldehyde evolution, and coloration. These deficiencies with the novel catalyst comprising an epoxy resin/inorganic salt acid where cure is effected between an ester polymer and hydroxyl containing polymer. For instance, hydroxyl functional polyesters can be crosslinked with dimethyl adipate, where methanol is liberated from the coating during cure as ester crosslinks form. These and other advantages of this invention will become more apparent by referring to the detailed description and the illustrative examples.